This invention relates to a process for preparing a transparent polymeric product free from strain and bubbles and having improved properties.
It is well known that transparent plastics are being substituted for inorganic glass in certain fields of application due to their light weight, improved workability and impact resistance. Further, in certain other fields of application, such plastics have become popular because of unavailability of other suitable materials.
However, only a few transparent plastics are known which are improved in the various properties such as mechanical strength, transparency, weather resistance and the like and have good appearance required for general-purpose plastics. One class of transparent plastics which is commercially used is acrylic resin, particularly polymethylmethacrylate, because it has a relatively high rating in the various properties stated above.
In general, one of the serious defects of plastics is low wear property and surface hardness; the surface thereof is easily scratched in comparison with inorganic glass. Another defect of the plastics is that they are inferior to inorganic glass in heat and chemical resistance. The polymethylmethacrylate with the properties stated above is not perfectly satisfactory.
Recently, a polymer of diethyleneglycol-bis-allylcarbonate represented by the formula ##STR1## has been developed. This is a transparent plastic having highly improved wear property and resistance to heat, impact, and the like. However, although the polymer of diethyleneglycol-bis-allylcarbonate has the above excellent physical and chemical properties, it can not be shaped by molding because of its high heat resistance. Therefore, to obtain a satisfactory polymeric product the monomer must be cast polymerized. However, in order to carry out the cast polymerization effectively the polymerization time must be shortened (i.e. the polymerization rate must be increased) and this creates problems in that strain, optical anisotropy and high residual stress in the product give the product defects such as lowering of optical properties and mechanical strength.
Further, it is also a serious problem that the polymeric product usually has bubbles which are formed by the vaporization or decomposition of the monomer or by expansion of entrained air due to the local temperature increase based on accumulation of heat generated during polymerization. In order to avoid such defects, the cast polymerization should be carried out by initially heating the monomer at a relatively low temperature of from 40 to 50.degree. C. and the gradually elevating it over several to a dozen or so hours to complete the polymerization. Therefore, one cast polymerization run normally takes about 24-40 hours to produce a product about 0.5 cm thick and about several tens to several hundreds of hours for a product having a thickness above about 0.5 cm. Such low efficiency in cast polymerization checks the application of the product for general-purpose plastics although a plastic such as a polymer of diethyleneglycol-bis-allylcarbonate has improved physical properties.